1. Field of the Invention
The invention relates to gasoline engines in which the expansion ratio and fuel efficiency at partial loads can be substantially enhanced by adapting the size of the compression clearance space to the momentary cylinder charge. The concept disclosed here is believed to be most suitable for application with electronically controlled four-stroke gasoline engines, although some of its elements can be utilized in the general technical field.
2. Description of the Prior Art
Engines with variable compression ratio were originally built for fuel testing and research purposes. They are single-cylinder and stationary; their mode of varying the compression ratio does not lend itself for use on modern multi-cylinder vehicular engines.
A multitude of patents emerged in the last six decades proposing solutions to the problem of varying the compression ratio by changing the height of two-part pistons so as to reach a substantially constant maximum pressure in the cylinder. U.S. Pat. No. 2,742,027 takes advantage of the inertial and hydraulic forces acting upon parts of the reciprocating piston assembly for incrementally raising a telescoping piston crown, conveniently using engine oil from the lubricating system as a hydraulic fluid which is forced through one-way valves in and out of the space between the two parts of the piston. After reaching the appropriate height corresponding to the size of the cylinder charge, the excess fluid is discharged into the oil pan through a spring-loaded pressure relief valve when the cylinder pressure reaches its peak.
A number of patents follow on the same principle, improving on the design of the valves and other mechanical parts, adapting the piston for two-cycle engines (U.S. Pat. No. 3,200,798), adding compensation for engine speed (U.S. Pat. No. 3,205,878), oil filtering means (U.S. Pat. No. 3,417,738), bubble removal (U.S. Pat. No. 3,656,412), improving access to the hydraulic valves (U.S. Pat. No. 4,241,705), adding temperature-sensitive variation (U.S. Pat. No. 4,979,427).
U.S. Pat. No. 4,687,348 takes a different approach--it provides a choice between two piston heights (two pre-set compression ratios) by means of an excentric piston pin bearing; switching between the two positions is again accomplished by hydraulic means using pressurized engine oil. U.S. Pat. Nos. 4,721,073, 4,809,650, 4,834,031, 4,864,975 and 4,934,347 follow, improving on the same principle.
U.S. Pat. No. 4,864,977 uses an external source of hydraulic pressure and external timing equipment for switching between two pre-set heights of the two-part piston.